During the war, farm hands were drafted or enlisted, the farmers who were left were making money, and equipment manufacturers were told that making tractors was a patriotic duty. As a result, when the war ended, the horses that remained on American farms lost their jobs.

After the war, sales of tractors skyrocketed. In the 1920s, only a few large farmers owned tractors. In the 30s, farmers were strapped for cash by the Depression, just like the rest of the nation. But in the 40s, those restraints were gone. So, when rationing ended, so did the careers of most draft animals.

Agricultural historian Bruce L. Gardner has charted the number of horses and tractors on farms in his book American Agriculture in the Twentieth Century. He conservatively estimates that one tractor would replace about five horses or mules during the early part of the century. The number of horses peaked at just over 25 million animals around 1920. About that same time, the number of tractors began rising and peaked at just under 5 million in the late 60s and 70s. The turning point  when the amount of tractor power overtook the amount of horse power on American farms  was 1945.

Mechanization changed almost everything on the farm. There were profound changes in the use of the land. To put it simply, horses run on oats that the farmer had to grow. Tractors run on gasoline that the farmer has to buy. In 1915, an estimated 93 million acres of cropland (27 percent of the total harvested acres) were used to grow feed for horses and mules. By 1960, that acreage had dropped to 4 million, freeing land for cash crops.

In large part because of mechanization, today's farms are much larger than they were before the 1940s, but farmers are much less self-sufficient. They now rely on external providers to supply the inputs they need to grow their crops or livestock.